A detailed examination of each step of the reaction sequence in the photochemical sacrificial hydrogen generation system consisting of [Ru(bpy)(3)](2+)/ascorbate/[Co(DPA-bpy)OH2](3+) was conducted. By clearly defining quenching, charge separation, and back electron transfer in the [Ru(bpy)(3)](2+)/ascorbate system, the details necessary for evaluation of the efficiency of water reduction with various catalysts are provided. In the particular Co(III) catalyst investigated, it is clear that the light induced catalytic process is considerably less efficient than the electrocatalytic process. A potential source of catalyst inefficiency in this system is reduction of the products formed in oxidation of the sacrificial electron donor. The data provided for excited state quenching and charge separation in this particular aqueous system are meant to be used by others for thorough investigation of the quantum efficiencies of other aqueous homogeneous and nanoheterogeneous catalysts for water reduction.